In data acquisition systems, information about a plurality of parameters is often obtained by simultaneously deploying numerous sensors. Various factors such as physical restrictions limiting access to the sensors, their remoteness, and the cost of cable, require acquisition and processing of signals from the sensors at a local unit before transmission to a distant master data processing unit. Moreover, in many applications the acquired signals must be monitored and processed locally to assure timely activation of local process controls and then temporarily stored to facilitate sequential transmission of data signals from all of the local sensors to the master processing unit.
The transmission format between the local unit of the acquisition system and the master unit is determined by consideration of such factors as limited frequency responses available through the connecting cables and differences in the data acquisition rates of various sensors. Systems such as that disclosed in Vancsa, U.S. Pat. No. 4,349,821 and in Nelligan, U.S. Pat. No. 4,012,712 suggest various data multiplexing and local signal storage schemes, respectively, in endeavors to enhance transmission resolution of signals acquired over parallel channels from several sensors. The electrical links providing data transmission channels between a local unit and the sensors used in monitoring process controls in such fields as submerged petroleum production facilities, for example, often require different signal output formats to accommodate for differences in the relative proximity of sensors to the local data acquisition station (e.g., signals transmitted over longer electrical links are more susceptible to picking up noise and are influenced by the greater link resistence), and to take into consideration special transmission requirements imposed on interconnections (e.g., inductive couplings) between some of the links and the local acquisition unit. There is a need therefore, for a data acquisition system which accommodates differences in possible operating modes and characteristics of various sensor channels. Moreover, while it may be desirable in particular applications to locally process data signals before transmission to a central distant master processing unit, it is highly desirable that in the event of failure of the processing stage at the local unit, the data signals be reliably transmitted directly to the master unit in a format which can be accurately decoded.